Harris or other Heavier Tweeds - Practical?

I'm considering having a suit made up in Harris Tweed, but the heaviest cloth I've ever worn is 14 ounces. Harris Tweed is 19-21 ounces.

With modern heating present pretty much everywhere, I'm wondering if this cloth is practical for wear in the city. Basically I don't want to be uncomfortable and sweating bullets wearing Harris Tweed in a heated building or home.

In the city, working in a heated building, no need for an entire suit made of tweed unless you suffer from chronic chills.
This is where you use your best judgement because only you know your body's needs and limitations.

I have a tweed sports jackets for the winter in 21oz Johnston of Elgin cloth

If you want a tweed jacket for the city to be worn in the autumn and winter, I think 17/18oz would be enough or 20+ if you want it for very cold weather. Do not get a suit in cloth that heavy, I would go for trousers in a lighter cloth (e.g. 16oz).

All depends on your location, you may be able to get away with a heavier cloth in some cities and in others, you it may be too heavy.

J.G. Hardy have Harris available in 13-14 oz. I think a few of the other cloth merchants have "light" weight Harris in the same weight. You might also try P&H Glenroyal, Worsted Alsport, or some of the lighter-weight tweeds from Islay Woollen Mills...

The material is great. I understand your fondness for it. Consider an overcoat and a sportcoat. An entire suit would be too much, both in terms of comfort in the heated office and in terms of appearance.

I think Harris is too loose a weave for trousers. There are some other harder tweeds like cheviot (P&H Glenroyal) that are ok for suits. I like the look and have a brown herringbone tweed suit, but I only wear the full rig about twice a year since it's so heavy.. and I get hot. I wear the jacket as a sport coat more often.